Recently, a variety of optical disks on/from which a large quantity of information can be recorded/reproduced have been developed. An example of large capacity optical disks is a double-sided optical disk in which two optical disk pieces are attached together and information can be recorded/reproduced on/from either side of the disk. However, in the field which frequently requires random access, for example, recording mediums for use with computers or game machines, there is a demand that the optical disk has a large recording capacity while any data in the disk can be accessed without turning the disk over.
Therefore, as an optical disk on which a large quantity of data can be recorded and in which random access can be performed, a multilayered optical disk in which there are two or more recording layers and information can be recorded/reproduced on/from one side of the disk has been suggested. FIG. 29 shows an example of such a disk.
FIG. 29 is a cross-sectional view of an optical disk 2300 having two recording layers. In FIG. 29, reference numerals 2101 and 2102 respectively denote transparent first and second substrates of polycarbonate or the like, reference numeral 2103 denotes a first recording layer, reference numeral 2104 denotes a semitransparent reflection film which transmits or reflects a laser beam incident on the first substrate 2101, reference numeral 2105 denotes a second recording layer, reference numeral 2106 denotes a reflection film which reflects a laser beam 2301 incident on the first substrate 2101, and reference numeral 2107 denotes an adhesive for attaching the substrates 2101 and 2102 which has the property of transmitting light. This structure makes it possible to perform a recording/reproducing operation on either of the recording layers 2103 or 2105 with the laser beam 2301 incident on the disk from the side of the substrate 2101.
Next, FIG. 30 is referenced. FIG. 30 is a diagram illustrating optical characteristics of the optical disk 2300 having the two recording layers 2103 and 2105 shown in FIG. 29. Here, a phase change material is considered to be used for the recording layers. A recording operation on the phase change material is performed by irradiating a disk being rotating with a light beam from a semiconductor laser so as to heat and resolve a recording layer of the disk. The temperature that the recording layer reaches and a process for cooling the recording layer vary depending on the strength of intensity of the light beam, thereby causing a phase change in the recording layer.
When the intensity of the light beam is strong, the recording layer is rapidly cooled from its high temperature state so as to be brought into an amorphous state, and when the intensity of the light beam is relatively weak, the recording layer is gradually cooled from a middle or high temperature state so as to be crystallized. Portions which are brought into the amorphous state are generally referred to as a mark, and crystallized portions between marks are generally referred to as a space. Binarized information is recorded on the mark and space. In the reproducing operation, the recording layer is irradiated with a weak light beam to such an extent as not to undergo a phase change and a difference in quantity of reflection light between the mark and space portions is detected so as to obtain a reproduced signal.
As shown in FIG. 30, the first recording layer 2103 is designed such that the occupancy of a transmission coefficient is high, i.e., a high transmission coefficient design, and the second recording layer 2105 is designed such that the occupancy of an absorption coefficient is high, i.e., a high absorption coefficient design. In this manner, generally in the dual-layer optical disk having two recording layers, the first and second recording layers 2103 and 2105 have different characteristics.
As described above, since the recording layer characteristics of the first and second recording layers 2103 and 2105 are different, optimal recording/reproducing conditions of the first and second recording layers are different. As an example, the reproducing condition of the second recording layer 2105 is described.
In general, in order to record/reproduce data, a recordable optical disk requires data management for each sector, regardless of a single-layer disk or a multilayered disk. Therefore, it often happens that guiding grooves for a tracking operation of a servo means are formed in a disk production process and along with this, address information of sectors is formed as pits. A sector structure of the second substrate 2102 of the dual-layer optical disk is illustrated in FIG. 31.
In FIG. 31, reference numeral 2102 denotes a second substrate, reference numeral 2302 denotes a groove track, reference numeral 2301 denotes a land track between grooves, reference numeral 2303 denotes an address region including concave and convex pits, reference numeral 2304 denotes a data region, and the address regions 2303 and the data regions 2304 are provided in both the land track 2301 and the groove track 2302.
Next, FIG. 32 is referenced. FIG. 32 provides more detailed illustration of the vicinity of the address region 2303 of the second substrate 2102. As illustrated in FIG. 32, the groove track 2302 sinusoidally oscillates at a constant frequency and the address regions 2303 are provided in both the land track 2301 and the groove track 2302. The address region 2303 is a region exclusively used for reproduction and is usually in a crystal state. This results from the fact that the optical characteristic of the recording layer is unstable immediately after the recording layer is formed, and therefore the address region 2303 and the data region 2304 are irradiated with a laser beam so as to be brought into a uniform crystal state.
In the case of reproducing data from the address region 2303 of such a dual-layer optical disk 2300, when the data is reproduced from the first recording layer 2013, as illustrated in FIG. 30, since the first recording layer 2013 has a reflection coefficient of 10% at the crystal state, up to 10% of the quantity of light returns to a photodetector. However, when the data is reproduced from the second recording layer 2015, the light is required to pass through the first recording layer 2013 having a transmission coefficient of 50% in both directions, and therefore, in fact, only 3.75% of the quantity of the light returns to the photodetector, although the reflection coefficient of the second recording layer 2105 is 15%, which is greater than that of the first recording layer. Accordingly, in order to reproduce data from the address region at an equivalent signal-to-noise ratio, irradiation power of a light beam in the case of reproducing data from the second recording layer 2105 is required to be set so as to be larger than that of the light beam in the case of reproducing data from the first recording layer 2103. However, in the case where the irradiation power capable of reproducing data from the second recording layer 2105 is preset, when a focusing operation of a servo means or a tracking operation of the servo means is erroneously performed on the first recording layer 2013, there is a risk that data recorded on the first recording layer 2013 might be erased due to the excessively large irradiation power.
On the contrary, in the case of setting the irradiation power sufficiently small as to reproduce data from the first recording layer 2013, when the address information cannot be reproduced, there is a risk in determining that the servo means is performing an operation on the second recording layer 2105 and immediately increasing the irradiation power. This is because there are some cases where the servo means is actually performing an operation on the first recording layer 2013, rather than the second recording layer 2105. Therefore, it is necessary to check on a number of things, such as optimization of the reproducing conditions, reproducing operations on a plurality of regions, etc., before increasing the irradiation power.
Further, it is not possible to securely discriminate between the first and second recording layers 2013 and 2015 by observing a focus error signal or a focus sum signal in which two elemental signals included in the focus error signal. As described above, when there is a difference between the first and second recording layers 2013 and 2015 with respect to the irradiation power used for a reproducing operation, it is necessary to discriminate between the first and second recording layers 2013 and 2015 more securely.
An objective of the present invention is to provide a recording medium in which a plurality of recording layers are securely identified in a short period of time.